1. Field of the Invention
The invention relates to interspinous stabilizers to be fixed to two vertebrae.
2. Description of Prior Art
A stabilizer of this type which includes two anchor members to be fixed to the vertebral pedicles of two vertebrae and a rigid body joining the two members together is known in the art. To treat degeneration of the disk, the stabilizer is used to immobilize the two vertebrae rigidly relative to each other and thereby to relieve the associated intervertebral disk. However, the stabilizer referred to is not entirely satisfactory. The surgery to fit the stabilizer requires access to the vertebral pedicles, or even the disk. It is therefore necessary to penetrate deeply into the body of the patient, which makes the operation more serious. What is more, because the two vertebrae are totally immobilized relative to each other, the disk is no longer loaded and continues to degenerate. Furthermore, the anchor members which are attached to the pedicles make them fragile and cause partial modification of the articular processes. Finally, fitting the anchor members requires delicate aiming in the pedicles so as not to exit from them and touch the dura mata, for example.
U.S. Pat. No. 5,645,599 discloses an interspinous stabilizer having members which are to be anchored to the spinous processes and which are joined together by a U-shaped leaf spring in a plane perpendicular to the geometrical generatrix of the spring. The stabilizer is adapted to be fixed to the spine so that the generatrix is parallel to the right-left direction referred to the body of the patient, the base of the U-shape being on the same side as the vertebral bodies and bearing against one of the vertebrae. Thus the stabilizer is entirely housed between the processes. It follows that the stabilizer is necessarily very small. This complicates its manufacture or makes it obligatory to give it a very simple shape to make it very rigid, which generates new risks of degeneration of the disk, as already mentioned.
An aim of the invention is to provide a different type of stabilizer and in particular one that is easy to manufacture an can be of low stiffness.
To achieve this object, the invention provides an interspinous stabilizer including two members adapted to be anchored to spinous processes of respective vertebrae and a body extending in a direction of alignment of the members, the body being compressible in the alignment direction and from a given configuration by the application of a force, the body being adapted to revert to the given configuration spontaneously after the force is removed, the body comprising a leaf spring having a geometrical generatrix, characterized in that the anchor members are adapted to fix the stabilizer to the processes so that the generatrix extends substantially from front to rear with reference to the body of the patient.
The stabilizer therefore allows some mobility of the two vertebrae relative to each other by partly reproducing the biomechanics of a healthy intervertebral disk. Moreover, the disk continues to be partly loaded, although the stabilizer relieves it of a great part of the stresses ordinarily imposed on it. Degeneration of the disk can therefore be slowed down or even stopped. The stabilizer preserves the integrity of the tripod articulation of the vertebral unit: the disk and the two posterior articulations and the associated connections at the level of a vertebra, namely the pedicles and the blades. Fitting the stabilizer to the spinous processes is a simple procedure. Also, the integrity of the protection of the dura mata is certain to be preserved.
What is more, orienting the generatrix in the front-rear direction enables the stabilizer to be extended laterally beyond the processes. Its volume can therefore be increased to make it both easier to manufacture and less rigid, if necessary, in order to limit the risk of degeneration of the disk.
The stabilizer according to the invention can also have one or more of the following features:
the body comprises two leaf spring parts extending parallel to each other in the alignment direction;
each part forms at least one U-shape in a plane perpendicular to the generatrix;
the body comprises at least one leaf spring part forming at least two successive U-shapes in a plane perpendicular to the generatrix and oriented alternately in opposite directions;
the spring has at least two sections of different thickness;
the body includes two leaf springs bearing on each other;
the leaf spring forms a closed loop;
the loop is elliptical;
the spring is thicker near a major axis of the ellipse than near a minor axis of the ellipse;
the body includes at least one viscoelastic material member;
the member is inside the loop;
the body includes two viscoelastic material members near respective ends of a major axis of the ellipse; and
the or each member has a cylindrical face in contact with a face of the spring.
The invention also provides an interspinous stabilizer including two members adapted to be anchored to spinous processes of respective vertebrae and a body extending in a direction of alignment of the members, the body being compressible in the alignment direction and from a given configuration by the application of a force, the body being adapted to revert to the given configuration spontaneously after the force is removed, in which the body has slots disposed to render the body compressible in the alignment direction.
The stabilizer according to the invention can also have one or more of the following features:
the body is cylindrical and is hollowed out along an axis of the cylinder;
the slots form at least one series of n adjacent slots symmetrically disposed about an axis of the cylinder and each slot subtends an angle greater than 180xc2x0 about the axis;
it includes at least two bodies which are parallel to each other in the alignment direction;
at least one of the anchor members (2) comprises two toothed jaws (23) mobile axially towards each other to form a clamp.